1. Field of the Invention
The present invention relates to a four-cycle engine that is intended for use in a vehicle, such as for example a snowmobile or a three-wheeled vehicle. More particularly, the present invention relates to a four-cycle engine having an improved induction system that complies with the strict emission control regulations currently developed in the United States without having a negative effect on the balance of the vehicle. The engines disclosed herein are described in connection with a snowmobile. The present invention, however, is not intended to be so limited; rather, it is contemplated that the engines described herein may be used in motorcycles, all-terrain vehicles, and various watercraft.
2. Description of Related Art
Snowmobiles are used for cross-country travel, during which it is frequently necessary to negotiate steep ascending and descending gradients, which requires powerful engines. Snowmobiles are used for both leisure-time pursuits as well as in a work environment. With this in mind, various demands are placed on the engine characteristics with respect to engine speed and torque. Known snowmobiles include a frame. Two steerable spring-mounted skis are installed on the front of the frame. A track driven by the engine is located on the lower rearward end of the frame. The track serves to propel the snowmobile over snow or ice covered ground. The engine and the track are usually connected by way of a continuously variable transmission (CVT), and a positive connection between the engine and the CVT. The positive connection is typically a centrifugal clutch that is integrated into the CVT.
At present, two-cycle engines are typically used to drive snowmobiles because these engines are capable of delivering a relatively large power output for a small installed size and low weight. Two-cycle engines, however, emit a considerable quantity of unburned hydrocarbons and other pollutants into the environment in the form of exhaust gas. The hydrocarbons and other pollutants are formed within the engine cylinders during the combustion process when the cylinder is inadequately flushed, and as a result of the lubricating oil that is added to the fuel.
Considerable structural and design modifications must be incorporated into the two-cycle engine to comply with current and ever more rigorous emission control regulations, which results in higher production costs. These modifications may include fuel injection and the use of catalysts. Furthermore, costly design features must be incorporated when the engines are used in snowmobiles to ensure that noise emissions are comparable to those of four-cycle engines.
One known snowmobile having a four-cycle engine is manufactured by Redline. The engine is a single overhead cam (SOHC), V-twin internal combustion engine that develops approximately 90 kW of power. The engine was originally designed for use in a motorcycle. These snowmobiles, which are up-market vehicles that are marketed under the brand name “954 Revolution,” are sport machines having a tubular frame. As such, these vehicles are only suitable for small-scale production. Due to predetermined minimum track width, the CVT is always remote from the longitudinal axis of the snowmobile. This arrangement is problematic especially for V-twin designs. If the center of gravity of the engine and the center of gravity of the CVT are on the same side of the vehicle, this would have a negative impact on the balance of the snowmobile and handling is made more difficult.
If the center of gravity of the engine (without auxiliary units) is arranged on the longitudinal axis of the snowmobile or on the opposite side of the vehicle relative to the center of gravity of the CVT, such an arrangement would require a relatively long drive shaft between the engine and the CVT. This arrangement, however, would generate undesirable oscillations within the drive train, which could result in a reduction of the service life or the destruction of the drive train. Furthermore, an engine in a snowmobile should be located to the rear as far as possible in order to locate its center of gravity as close possible to the track, which enhances the snowmobile handling and improves driving dynamics. This arrangement is not possible in the Redline design because the engine would collide with the steering rod.
Maximumsled also produces a snowmobile under the brand name “Venom” that is also based on a motorcycle engine. This snowmobile suffers from many of the same problems discussed above.
Large-scale production snowmobiles are typically manufactured from a sheet metal profile frame that is preferably of aluminum. A snowmobile of this kind is sold, for example, by Yamaha under the brand name “RX-1” and “RX-1 ER.” This snowmobile is powered by a four-cycle, four-cylinder, in-line, carburetor-type motorcycle engine that is installed transversely to the longitudinal axis of the vehicle. The engine has a dry-sump lubrication system, and develops approximately 107 kW of power. This engine has a relatively high nominal engine speed. As a result, additional reduction gearing has to be installed between the crankshaft and the drive pulley of the CVT. This engine has numerous drawbacks including a greater installed length and a greater weight. Furthermore, the exhaust runs beneath the tank and beneath the seat to the rear of the snowmobile. This produces a significant buildup of heat beneath the tank and the seat.
Published U.S. patent application Ser. No. 09/925,522 to Yatagai et al. discloses a snowmobile four-cycle engine arrangement. Yatagai discloses a four-cycle engine arranged in an engine compartment formed in the front body of a snowmobile. The crankshaft of the engine is laid substantially parallel to the body width of the snowmobile. The engine has a cylinder case inclined in a forward direction. The engine has a dry sump oil supplying system and an oil tank separate from the engine. This engine arrangement has several drawbacks. First, the cylinders are inclined in the forward direction. The turbocharger and oil tank are located in front of the engine. With this arrangement, the center of gravity of the engine is positioned relatively far away from a center point of the vehicle and the track. This adversely impacts the handling and maneuverability of the snowmobile. Second, the snowmobile is typically operated in severe working conditions (temperature changes between +15° C. to −40° C., ice formation, etc.). The water pump and alternator are belt driven. The belt is prone to failure.